FIG. 18 is a diagram illustrating the configuration of a conventional duplexer. This conventional duplexer 81 includes a transmitting filter 82, which is connected between a common terminal Ant and a transmitting terminal Tx, a receiving filter 83, which is connected between the common terminal Ant and a receiving terminal Rx, and a matching circuit 84. When the duplexer 81 is used in a mobile phone for example, the common terminal Ant is connected to an antenna 94. The transmitting terminal Tx is connected to a transmitting circuit 93, and the receiving terminal Rx is connected via a balanced output filter 91 to a receiving circuit 92.
In FIG. 18, arrow Y1 denotes the flow of a transmission signal. Since the pass band of the transmitting filter 82 differs from the pass band of the receiving filter 83, when the transmission signal that is input into the transmitting terminal Tx passes the transmitting filter 82, as indicated by the arrow Y1, it does not flow to the receiving filter 83 but is output from the common terminal Ant to the antenna 94. Then, the transmission signal is radiated as an electromagnetic wave from the antenna 94.
Arrow Y2 denotes the flow of a reception signal. The reception signal received by the antenna 94 passes the matching circuit 84 and the receiving filter 83, as indicated by the arrow Y2, and is output from the receiving terminal Rx. The balanced output filter 91 converts the received signal into a differential signal (balanced-unbalanced conversion), and outputs it to the receiving circuit 92. This is the ideal signal flow within the duplexer.
However, in real duplexers, the entire transmission signal that is output from the transmitting filter 82 is not necessarily output from the common terminal Ant to the antenna, and a portion of it flows into the receiving filter 83 and reaches the receiving terminal Rx. Arrow Y3 indicates the flow of the transmission signal (transmit/receive leakage signal) that leaks in this manner to the receiving filter. Because of this transmit/receive leakage signal, there was the problem that the reception signal that is input from the receiving terminal Rx into the receiving circuit includes noise, and as a result, the receiving properties deteriorated.
Here, the transmit/receive isolation (referred to simply as “isolation” below) is used as a value that indicates the degree to which the transmit/receive leakage signal leaking from the transmission signal, which is input from the transmitting circuit to the transmitting terminal Tx, to the receiving filter 83 and reaching the receiving terminal Rx is suppressed.
With the duplexer 81 indicated in FIG. 18, the balanced filter 91, which is connected between the receiving terminal Rx and the receiving circuit 92 has the function of improving this isolation. As another example, a configuration has been disclosed (see for example Patent Document 1 listed below), where a shield electrode is provided in order to eliminate unnecessary electromagnetic coupling within the antenna branching filter, which is one of the causes of transmission signal leakage from the transmitting circuit to the receiving circuit.    Patent Document 1; Japanese Laid-Open Patent Publication No. 2006-60747
However, providing the above-noted balanced filter 91 or shield electrode leads to an increase in the number of parts, to higher costs and to greater complexity. Thus, with conventional duplexer configurations, it was difficult to improve the isolation while avoiding an increase in the number of parts, higher costs and greater complexity.